1. Field of the Invention
This disclosure relates to an apparatus for transporting a print head across a platen and for providing signals to the print head to generate characters.
2. Description of Prior Art
Computer systems are becoming significantly less expensive, making them cost effective for an increasing range of applications. Formerly, the primary cost of a computer system was attributable to the electronic circuitry required to provide the processing capabilities. The cost of peripherals, such as printers, mass storage units, CRT terminals, modems, etc. and the cost of software were minor when compared to the cost of the central processing unit. However, the recent advances in semiconductor electronics have caused a precipitous decline in the cost of processing logic, with the result that computer systems are now suitable for use in new markets.
The major costs of present day computer systems are in the cost of peripherals and software. Software packages, if able to be marketed as a standard product not requiring major changes for each customer, may be sold at a low cost since the cost of development of the software is distributed among many customers. Computer peripherals, however, cannot achieve similar reductions in cost. Although the marginal cost to install an additional copy of an existing software package is very low, the marginal cost to manufacture a computer peripheral is considerable. Computer peripherals are typically a hybrid between mechanical and electrical engineering technologies. The mechanical aspects of a peripheral, such as precision-formed components, and the cost of assembly, have been fundamental limitations on the cost of such peripherals. Nevertheless, considerable research is being performed to develop less expensive computer peripherals.
Commercially available computer printer peripherals range from typewriter-based devices printing 10 or 15 characters per second to laser-based printers able to print many pages per second. Computer printers can be divided into three fundamental classes:
(1) Character printers, such as computer-driven typewriters, where each character is individually formed or printed;
(2) Line printers, wherein an entire line of print is assembled simultaneously, e.g. by providing a drum rotating before a sheet of paper in which at each print position there is a corresponding segment around the drum containing a print element for each printable character; and
(3) Page printers, such as laser-based printers or xerographic printers, which form an entire page of print at one time.
The most inexpensive type of printer is the character printer. Such printers have been used for many years as typewriters and in such applications as telegraphy. One of the least expensive of the character printers is the thermal dot matrix printer.
A thermal dot matrix printer requires a special paper, which may be selectively discolored by application of heat. The thermal print head contains several wire elements, each of which may be heated as desired. The print head is moved across the paper in a straight line and the wire elements are controllably heated to form the desired characters on the sensitized coating on the paper. Typically, there may be 7 wire elements in a thermal print head, thereby allowing 7 rows of "dots" to be formed as the print head is drawn across the paper. The print head is controlled so that each wire element may be turned on or turned off at, for example, five discrete positions for each character position in the print line. In this manner, the interaction of the 7 rows of print head elements with the 5 potential columns in a character position form a 5.times.7 matrix of positions in which dots may be selectively formed by the print head to create the desired character. Such thermal dot matrix printers are used where an especially inexpensive printer is desired and a slower print rate, lower quality printing, and requirement for sensitized paper are acceptable trade offs when compared to the low cost of the printer.
A major mechanical assembly of such a printer is the carriage assembly, which moves the print head across a paper under control of logic circuitry. Such circuitry also controls the heating of the print head wire elements. The carriage assembly has several requirements. First, it must permit the head to move bi-directionally, i.e. back and forth across the paper. Second, the carriage mechanism must accurately locate the print head during its movement across the paper so that predictably uniform characters may be formed. Third, the carriage mechanism should allow the print head to be easily removed and replaced inasmuch as individual wire elements in the print head tend to "burn out". Fourth, the carriage mechanism must allow the paper to be inserted between the print head and platen in a convenient manner.
There have been several approaches to the design of thermal dot matrix printer carriage assemblies. Certain of such assemblies have been provided with a lead screw which is threaded through a portion of the carriage assembly. The screw is fixed with respect to the carriage assembly and extends in front of the platen. By appropriate rotation of the screw, the carriage assembly is transported across the line to be printed. Additional guides for the carriage assembly are typically required in order to insure that the movement of the carriage assembly is precisely performed and that the print head remains in the desired alignment with the platen.
Another approach to the design of thermal dot matrix printers has been to provide a carriage assembly slidable on guide shafts, the shafts determining the alignment of the print head with respect to the platen. The assembly is moved via a flexible wire cable which is pulled via a motor. By appropriate cabling, the carriage can be moved back and forth in a precision manner.
Unfortunately, these two approaches to thermal dot matrix printers, and other such approaches in the prior art, have suffered from several limitations. For instance, auxiliary guide shafts are typically required to insure that the print head maintains a precise position with respect to the platen as it moves across it, and to insure that the print head has no "wobble" or other undesirable mechanical imprecisions in its movement. All such guide mechanisms contribute to the cost of the printer. Additionally, such parts must be precision formed, resulting in increased costs to manufacture them.
Inasmuch as any increase in mass of the carriage assembly which moves with the print head requires a more powerful, and therefore more expensive, drive motor than would be required for a lighter carriage assembly, any reduction in mass of the carriage assembly results in cost savings in other areas of the printer assembly also.
Accordingly, it is a primary object of the herein disclosed invention to provide a thermal print head carriage assembly which precisely positions the print head with respect to the platen as it is transported across the platen. An additional object is to provide for a carriage assembly which allows the print head to be easily removed for repair or replacement, and allows for the easy insertion of paper in the printer. It is also an object to provide a light, simple, inexpensive carriage assembly and drive mechanism.